The thermocline is the transition layer between the mixed layer at the
surface and the deep water layer. The definitions of these layers are
based on temperature.

The mixed layer is near the surface where
the temperature is roughly that of surface water.
In the thermocline,
the temperature decreases rapidly from the mixed layer temperature to the
much colder deep water temperature.

The mixed layer and the deep water layer are relatively
uniform in temperature, while the thermocline represents the transition zone
between the two.